[swift-evolution] [Proposal draft] Disallow Optionals in String Interpolation Segments

[Nate Cook]

> On Oct 3, 2016, at 5:49 PM, Kevin Ballard via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On Mon, Oct 3, 2016, at 03:18 PM, Jordan Rose wrote:
>>> 
>>> ...
>>> 
>> We had this at one point, but we took it out because people would forget to test the nil case. I think `?? ""` or `?? nil` really is the best answer here.
> 
> But you can't write that, unless you're dealing specifically with an Optional<String>.  If you try you'll get an error:
> 
> unnamed.swift:2:19: error: binary operator '??' cannot be applied to operands of type 'Int?' and 'String'
>     print("x: \(x ?? "nil")")
>                 ~ ^  ~~~~~
> unnamed.swift:2:19: note: overloads for '??' exist with these partially matching parameter lists: (T?, @autoclosure () throws -> T), (T?, @autoclosure () thro
> ws -> T?)
>     print("x: \(x ?? "nil")")
>                   ^
> This leads to writing code like "… \(x.map(String.init(describing:)) ?? "nil")" which is pretty gross.

I think that if we're going to add this warning we should make it possible to provide a string as an alternative. It seems like it should be possible to build a ?? operator with a (T?, String) -> _StringInterpolationSomething signature that works only in a string interpolation context.

There are some types that aren't trivially constructible, or don't have clear alternatives for the nil case. Other times it might just not make sense to build a new instance simply to turn it into a string. If we're going to make people provide an alternative for optionals in this otherwise simple-to-use construct, let's make it simple to do so.

This is undoubtedly a more complex approach that could be considered separately, but I think it would be a valuable part of how developers could transition their code.

Nate

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